1. Field of the Invention
The present invention relates to a permanent magnet motor equipped with a rotor consisting of a plurality of permanent magnets buried in a rotor core.
2. Related Art
There is disclosed a permanent magnet motor whose demagnetization resistance and efficiency has been improved by burying permanent magnets in a rotor core (see, for example, Japanese Patent Laid-Open No. 11-187597). FIG. 11 is a side view showing an end of a rotor of the permanent magnet motor as viewed along the insertion direction of a rotating shaft of the rotor before the rotating shaft is inserted. In FIG. 11, a rotor 2 consists of a rotor core 2a and rotating shaft (not shown), where the rotor core 2a is a generally pillar-shaped stack of steel plates cylindrical in outline. Near its outer circumference, the rotor core 2a has permanent-magnet-holding slots 5 corresponding to sides of an approximately regular octagon and each of the permanent-magnet-holding slots 5 contains a permanent magnet 4. The permanent magnets 4 are arranged in such a way that the S pole and N pole alternate with each other. A plurality of radially elongated slits 6 are arranged apart from each other along each of the permanent-magnet-holding slots 5 on an outer core 3 outside the permanent-magnet-holding slots 5. A rotating-shaft hole 8 is provided in the center of the rotor core 2a to accept the rotating shaft.
With the conventional permanent magnet motor described above, the slits 6 in the rotor core 2a are spaced at equal intervals to lead in and out magnetic flux of the permanent magnets 4 radially as well as to prevent magnetic flux (hereinafter referred to as armature reaction flux) generated by stator winding current from being bent along the circumference of the outer core 3.
However, when the plurality of slits 6 are arranged at equal intervals, radial magnetic flux distribution in the permanent magnets 4 has a trapezoidal profile. Consequently, geometric positional relationship between stator and rotor produces high cogging torque, increasing vibration.
Also, voltage induced by stator windings has a harmonic-rich distorted waveform, resulting in not only increased noise, but also increased core loss, which in turn results in reduced efficiency.
Furthermore, if the stator is driven by 3-phase sine wave AC current, only a fundamental wave component contribute effectively to torque while harmonic components produce torque ripple which increases vibration and noise.